Toxins have historically been considered to be stable preparations; however, with the preparation of combination vaccines and the production of antibodies or antitoxins by these vaccines, the characterization and subsequent stability of toxins and antitoxins has become more relevant. Examination of diphtheria toxin stability in the Vero cell test was implemented this year using a modified assay that included freeze-dried antitoxins and specific set potencies. Using this method, toxin dilutions were found to be very stable at -20 degrees C, but lost potency after thawing and refreezing. Studies on the reconstituted Reference Horse Antitoxin (RAT) for Diphtheria have indicated that this preparation is not stable for more than 6 months at 2-8 degrees C. This finding has been utilized to insure that stability of the toxin and antitoxin preparations used in the cell assay.